The following invention relates to a sensorless control circuit for use with a switched reluctance electric motor.
Switched reluctance stepping motors employ an iron core rotor having a plurality of poles and pairs of stator coils aligned on opposite sides of the rotor so as to create a magnetic circuit when the poles of the rotor are aligned with the oppositely placed coils. The reluctance of the magnetic circuit is lowest when a pole pair is aligned directly between a pair of opposed coils. By sequentially energizing pairs of coils, the rotor is caused to rotate in order to find the position of lowest reluctance.
Control circuits for such motors typically include an external sensor which is connected to the rotor to sense rotor position. This signal is used to maintain alignment between the coil switching sequence and the rotor position. Hall effect sensors have long been employed for this purpose. A mechanical sensor is shown in the U.S. Pat. No. 3,601,678, to Abraham, et al. In the Abraham patent, a mechanical emitter coupled to the shaft of the rotor provides position information for a reader head which in turn generates a feedback signal to a control loop which controls motor speed. Sensors, however, take up space, are costly, and typically cannot withstand the harsh environments for which switched reluctance motors are rated.
Various types of sensorless control circuits exist for DC brushless motors which employ permanent magnet rotors. Permanent magnet rotors generate back EMF in stator coils which various sinusoidally. Thus, determining the phase of the back EMF signal gives an indication of rotor position. An example of this type of control circuit is shown in Plunkett, U.S. Pat. No. 4,928,043 entitled BACK EMF SAMPLING CIRCUIT FOR A PHASE-LOCKED LOOP MOTOR CONTROL which is assigned to the same assignee as the present invention. The Plunkett invention discloses a sensing network for sensing the back EMF on an unenergized motor winding. This signal represents a phase error when compared with an optimum value and drives a voltage controlled oscillator which in turn generates timing signals for an inverter. However, because switched reluctance motors do not employ permanent magnets, there is no back EMF induced in an unenergized coil as a direct result of rotor rotation.
The inductance in the coils does vary as a function of rotor position. When a pair of rotor poles become aligned with a pair of stator coils, the inductance reaches a maximum. In the past, motor designers have attempted to build control loops utilizing some value of sensed inductance. However, this requires complex bridge circuitry which tends to have a high level of noise associated with it. Saturation and hysterysis effects also compromise inductance sensing techniques.